This invention relates to a method for producing a porous cross-linked polymer sheet by obtaining a porous cross-linked polymer from a water-in-oil type high internal phase emulsion (hereinafter occasionally referred to briefly as xe2x80x9cHIPExe2x80x9d), dehydrating the polymer, and then slicing the dehydrated polymer, and more particularly to a method for producing a porous cross-linked polymer sheet by compressing the polymer thereby removing water therefrom till the water content thereof falls below a prescribed level and thereafter slicing the polymer.
Regarding the production of a porous polymer formed of continuous cells of a minute and uniform pore diameter, the method for obtaining the polymer by subjecting an HIPE to cross-linking polymerization in the presence of a specific surfactant has been known. The term xe2x80x9cHIPExe2x80x9d as used herein is generally interpreted as referring to a disperse phase whose ratio to the total volume of a relevant emulsion exceeds 70 vol. % (K. J. Lissant, Journal of Colloid and Interface Science, Vol. 22, 462 (1966)). U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous cross-linked polymer by using such an HIPE (hereinafter referred to simply as xe2x80x9cHIPE methodxe2x80x9d).
This HIPE method is directed toward producing a porous cross-linked polymer by preparing the HIPE comprising (i) a polymerizable monomer mixture containing an oil-soluble vinyl monomer and a cross-linking monomer possessed of not less than two functional groups in the molecular unit thereof, (ii) a water phase accounting for a proportion of 90 weight %, preferably 95 weight %, and particularly preferably 97 weight % of an emulsion, (iii) a surfactant such as a sorbitan fatty acid ester and a glycerol monofatty acid ester, for example, and (iv) a polymerization initiator and heating the HIPE till it is polymerized and cross-linked. According to this HIPE method, a porous cross-linked polymer comprising reticularly continuous cells is formed by virtue of reversed-phase emulsion polymerization. The porous cross-linked polymer which is obtained by the HIPE method, therefore, possesses low density and such characteristic properties as water absorbing property, water retaining property, heat insulating property, and sound insulating property.
Though the porous cross-linked polymer which has such low density, namely a high hole ratio, is useful for a wide variety of applications, it is not satisfactorily efficient in transportation and storage on account of high bulkiness. With the object of eliminating this defect, International Publication of Unexamined Patent Application 96/40823, for example, discloses a method for producing a compressed porous sheet by preparing a porous cross-linked polymer by the HIPE method and compressing this polymer to one of several parts of the original thickness. The compressed porous sheet so produced possesses the nature of retaining the compressed state and, on exposure to a large quantity of liquid, quickly absorbing the liquid and reverting to the original thickness.
Other methods for producing a porous cross-linked polymer sheet by polymerizing an HIPE are described in Published Japanese Translation of PCT International Publication of Patent Application 6-509834, Published Japanese Translation of PCT International Publication of Patent Application 10-512168, WO97/27240, and WO97/45479, for example.
The method disclosed in Published Japanese Translation of PCT International Publication of Patent Application 6-509834, for example, comprises treating a polymerized water-in-oil type emulsion foam with a surfactant and an agent such as calcium chloride which fulfills the function of imparting hydrophilicity thereby rendering the foam hydrophilic and, while the foam is in used, causing the combination of the surfactant and the hydrate of calcium chloride to furnish the foam with a hydrophilic surface. When the foam happens to have a greater thickness than the product aimed at, the method contemplates slicing this foam. To be more specific, this method in this case resorts to a procedure which comprises slicing a porous cross-linked polymer retaining therein a residual water containing an emulsifying agent, an electrolyte, and an initiator all in a dissolved state in a total quantity 30-40 times the weight of the polymer material with a sharp reciprocating saw blade to obtain sheets 0.35 inch in thickness and then compressing such a sheet with rolls till it is dehydrated.
A similar mention is found in WO97/27240. This publication concerns a means to cure an HIPE and discloses a method for producing a foam by preparing an HIPE, supplying the HIPE to a continuous polymer film, allowing the HIPE on the film to cure, and winding the film into a roll. This invention further contemplates removing the film from the cured foam, slicing the foam into pieces of thin film, and dehydrating and drying the sliced pieces. When the HIPE can be shaped in a thin film, it is no longer necessary to slice the foam. By the method described in the publication, however, the continuous sheet adheres fast to the cured foam and turns eventually to a lining sheet for the foam. Thus, the foam is sliced after the HIPE has cured, and then the sliced foam is dehydrated, washed, and dried.
Then, according to the method disclosed in Published Japanese Translation of PCT International Publication of Patent Application 10-512168, since the HIPE foam which has been formed is packed with a residual water phase substance used in the preparation of the HIPE, this residual water phase substance is removed at least partly before the foam is further worked and put to use. This removal of the residual water phase substance is generally carried out after the foam has been sliced into a sheet measuring about 0.15-0.4 cm in thickness. The sheet thus obtained is compressed as with a porous nip roller to expel the residual liquid.
WO97/45479 teaches a process which comprises obtaining an HIPE having an anionic surfactant incorporated in advance therein, then hardening the HIPE to give rise to a foam, slicing the foam into a sheet 0.35 inch in thickness, then dehydrating the sheet, and compressing the dehydrated sheet to a thickness of 0.045 inch.
Generally, the slicing of a porous cross-linked polymer is effected by rotating at a prescribed speed an endless band knife which is stretched with driving pulleys opposed to each other across the knife. The endless band knife has a construction such that it is nipped by guides disposed on the vertically opposite sides thereof and adapted to allow the band knife to protrude slightly from the gap between the guides, with the result that the protruding band knife will slice the porous cross-linked polymer. When the continuously cured HIPE is sliced, therefore, the practice of continuously conveying a long cured HIPE strip to the band knife and slicing it into a sheet of a fixed thickness or successively slicing it into sheets of a prescribed thickness is generally adopted.
When the porous cross-linked polymer obtained by polymerizing the HIPE is sliced, however, the sliced pieces of the porous cross-linked polymer each in the form of a thin film adhere to the guides disposed as vertically opposed to each other across the band knife. This adhesion prevents the sliced pieces from smoothly moving on the guide faces and entails the possibility of suffering the porous cross-linked polymer to sustain a fold, a fracture, or a crack. The crack so inflicted degrades the product in quality. The prevention of this degradation of quality requires to lower the slicing speed and the decrease of the slicing speed forms a cause for retarding the production process.
Further, as disclosed in Published Japanese Translation of PCT International Publication of Patent Application 6-509834, the water phase during the preparation of the HIPE is incorporated therein such a salt as calcium chloride due to smooth progress of the emulsification. Since calcium chloride so added is contained in its unaltered form in the cured HIPE, is formed a cause for corroding the slicing band knife or the slicing device proper.
The generation of rust due to the corrosion possibly entails such a trouble as adhesion of rust to the sliced pieces. The porous cross-linked polymer which is obtained is generally used as a sound insulator and a heat insulator for absorbing sound and heat, as bases to be impregnated with such chemicals as an aromatizing agent and detergent, and as absorbents for oils and organic solvents. It can be also used as sanitary materials such as disposable diapers and sanitary articles, and as cosmetic articles and medical preparations which by nature directly contact the human body. When it is used particularly as a medical material or a sanitary material, the porous cross-linked polymer forming a finished product is required to have the surface thereof rendered incapable of appreciably stimulating the skin to the fullest possible extent with a view of preventing the contact from inflicting rush on the skin or imparting unpleasant feeling thereto. The security of the safety of using the material forms an important requirement.
This invention, therefore, intends to develope a method for producing a porous cross-linked polymer sheet with excellent efficiency which enables the porous cross-linked polymer to be sliced and transported without sustaining damage such as a fold or a crack and to be increased slicing speed without inducing the problem of the corrosion of the slicing device or the adhesion of rust to the sliced pieces.
This invention has originated in the discovery that by subjecting a porous cross-linked polymer produced by polymerizing the HIPE in an expected shape to a preparatory step of dehydration and a subsequent step of slicing, the shaped polymer is enabled to increase the mechanical strength of the material thereof and allow the material to be sliced and then transported without inducing damage such as a fold or a crack and that by performing this procedure, it is made possible to exalt the slicing speed, improve the slicing device in resistance to corrosion, and obtain a porous cross-linked polymer sheet incapable of inducing the sliced pieces to incur such trouble as adhesion of rust thereto. This invention has been perfected as a result.
Specifically, the object of this invention mentioned above is accomplished by the following items (1) and (2).
(1) A method for the production of a porous cross-linked polymer sheet, comprising a step for obtaining a porous cross-linked polymer by forming and polymerizing an HIPE, a step for dehydrating the porous cross-linked polymer, and a step for subsequently slicing the dehydrated porous cross-linked polymer.
(2) A method for the production of a porous cross-linked polymer sheet, comprising a step for obtaining a porous cross-linked former-stage polymer by forming an HIPE and subjecting the formed HIPE to a former-stage polymerization, a step for dehydrating the porous cross-linked former-stage polymer, a step of subsequently slicing the porous cross-linked former-stage polymer into pieces of a prescribed thickness, and a step for subjecting the porous cross-linked former-stage polymer to a latter-stage polymerization thereby obtaining a porous cross-linked polymer.
According to this invention, by performing the step for dehydration prior to the conventional step for slicing, it is made possible to prevent the porous cross-linked polymer from adhering to the blade and the guides provided in the slicer, allow the polymer to be sliced quickly, and as well prevent the production device from gathering rust and adhering to the porous cross-linked polymer owing to the removal of a salt.